Method for detecting cracks in metal bodies

ABSTRACT

A nondestructive method for detecting cracks in a metal body by selective corrosion of the surface portion within the cracks to enlarge them sufficiently so that they can then be detected by conventional techniques. Specifically, a crack enlarging solution which is an electrolyte containing halide ions is applied to the surface of the body to be tested. The solution is formulated so that it does not corrode the open surface portion of the body but is sufficiently reactive to corrode the surface portion within cracks. A conventional penetrating dye liquid is then applied to the same surface of the metal body and the cracks are detected in a conventional manner.

United States Patent Johnson et al.

1451 Mar. 28, 1972 [54] METHOD FOR DETECTING CRACKS IN METAL BODIES [73] Assignee: General Electric Company, Schenectady,

22 Filed: 06.31, 1969 211 Appl.N0.: 889,695

52 us. 01. ..23/230 R, 23/230 c, 252/408, 73/104, 250/71, 14816.1, 148/62 51 1111.01. ..G0ln 21/16, 00111 21/38, c23r 7/26 [58] r1610 61 Search ..23/230, 230 c; 252/408; 73/104; 250/71; l48/6.1, 6.14, 6.2

[56] References Cited UNITED STATES PATENTS 3,366,554 l/l968 Lindblad 3,490,873 1/1970 COl'l ..23/23o 2,007,285 7/1935 Schanffelc ..23/230 3,425,950 2/1969 Derbyshire, Jr. ..73/l04.

Primary Examiner-Morris O. Wolk Assistant Examiner-Elliott A. Katz Attomey-Charles T. Watts, Paul A. Frank, Jane M. Binkowski, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman [57 ABSTRACT liquid is then applied to the same surface of .the metal body and the cracks are detected in a conventional manner.

9 Claims, No Drawings METHOD FOR DETECTING CRACKS IN METAL BODIES This invention relates to a nondestructive method for detecting cracks in a metal body. In particular, the invention improves the sensitivity of standard dye penetrant crack detection techniques in metals.

Metal fatigue causes 75 percent of all the serious engine failures in jet aircraft today. If adequate inspection techniques were available, these failures could be prevented. The fatigue process is a slow one; early in the process small microcracks are nucleated either at grain boundaries intersecting the surface, at hard precipitate particles at or near the surface, or other surface flaws. These microcracks grow at a very slow but accelerating rate leading to a large crack which will eventually cause catastrophic failure if it remains undetected. Since the fatigue crack grows slowly, there is usually ample opportunity to detect the crack during inspection periods before it grows to a critical size.

Current crack detection techniques rarely are able to detect cracks smaller than 3 mils in depth, and in most circumstances to mils is usually the lower limit of detectability. With the more traditional structural alloys and conservative design, a crack can grow to much larger than this detection threshold before it becomes critical in size. But in the newer, high temperature alloys, which have a much lower toughness andare designed for much higher stress levels, the critical crack size begins to approach the limit of detectability. Under these conditions detection techniques of the prior art are not adequate.

A typical detection process widely used in industry utilizes penetrants known in the art which include various liquids mixed with a dye in combination with a developer or blotting agent. The use of such a liquid penetrant generally comprises application of the penetrant to the surface of the metallic specimen followed by a soaking period of at least about 30 minutes to permit penetration into cracks, if any. The excess penetrant is then removed usually with a suitable solvent. A developer or blotting agent which contains a powder such as ground silica or talcum, is then applied to the surface to provide a contrasting background to make the dye more visible and to blot the liquid dye out of the surface cracks by absorption. The pattern of blots shows the general location of cracks as well as other surface discontinuities. Some prior art methods use a fluorescent penetrant. Typically, this comprises applying to the surface of the test body the luminescent penetrant, removing the penetrant remaining on the surface generally by solvent cleaning, applying a developer, and then inspecting the surface of the body in darkness under fluorescigenous radiation such as ultraviolet light or black light for the portion of the penetrant which had penetrated into the surface openings.

These dye penetrant inspection procedures of the prior art have two important drawbacks. The effectiveness of the liquid penetrant depends on its wetting properties, i.e., the ability of the penetrant to be able to flow into a crack. Secondly, the cracks must be sufficiently wide to accommodate enough of the penetrant to give visible indications upon application of the developer. Tightly closed cracks or cracks closed at their mouths would therefore ordinarily not be detected by these dye penetrant procedures. This is a particular problem in the art since parts are inspected usually in an unstressed, cold condition which commonly leaves the cracks tightly closed.

The present invention overcomes these drawbacks of the prior art method by providing for the enlargement or opening of these difficult to detect cracks so that they can then be detected by conventional methods.

Briefly stated, the process of the present invention comprises initially applying to the surface of a metal body to be tested a crack enlarging solution which is an electrolyte solution containing halide ions. The electrolyte is formulated so as to be too weak to corrode the open surface portion of the body, but sufficiently strong to corrode the internal surface portion contained within the cracks to enlarge them sufficiently so that they can be detected by conventional techniques. If desired, the solution may contain an oxidizing agent to accelerate corrosion within the cracks. Alternatively,

corrosion within the cracks may be accelerated by controlling the bulk metal potential by external control with a potentiostat.

In the area of nondestructive testing for cracks, parts are frequently grit blasted to remove oxides and other contaminants prior to testing with a liquid penetrant. However, grit blasting smears the metal, and after a grit blasting operation the entire part is usually immersed into an acid etch to remove a thin layer of surface metal in order to remove the residue resulting from grit blasting. It is then rinsed off with cold water and is ready for testing by a conventional dye-carrying liquid penetrant. However, such preliminary treatment does not overcome or avoid the drawbacks of the prior art methods set forth above.

The present process detects cracks in a metal body by initially selectively corroding the surface portion within the cracks. By corrosion, it is meant herein the dissolution of the metal by its liquid environment. The cracks are then detected by a conventional liquid penetrants. Specific example of such penetrants are disclosed in U.S. Pat. No. 3,433,062.

in carrying out the process of the present invention, the surface of the metal body to be tested should be cleaned in a conventional manner to reduce background indications so that a clearer and sharper indication of cracks can be produced. Conventional methods of cleaning metals can be used such as by wiping the surface with a suitable organic solvent to remove oils and grease or by mild grit blasting to remove adherent surface contaminants such as paint and scale.

The crack enlarging solution is formulated so as not to corrode the open surface portion of the metal body but to corrode the metal surfaces within cracks in the body. For those bodies of metallic material which are generally known in the art as the type that exhibit crevice corrosion, the crack enlarging solution is always an aqueous acid solution containing halide ions, and its specific formulation is determinable empirically. Representative of these materials are austenitic alloy steels such as stainless steels, iron-base super alloys and the nickel-chromium-iron alloys such as those sold under the trademark of lncoloy.

To provide the necessary acidity as well as the halide ions in forming the crack enlarging solution for this type of metal, a halogen acid is preferably used, i.e., hydrochloric, hydrobromic, hydroiodic or hydrofluoric acid. Hydrochloric acid produces the best results and is preferred. If desired, the halide ion content of the crack enlarging solution can be provided or increased by the use of a soluble metal halide salt. Representative of such salts are alkali halide salts such as sodium chloride, potassium chloride and sodium iodide. Where a metal halide salt is used, the acidity of the electrolyte solution, if desired, may be provided by an acid other than a halogen acid. Representative of the acids useful with a metal halide salt are sulfuric acid and nitric acid.

Upon application of the solution to the surface of the metal body to be tested, the halogen acid constituent acts to corrode the surface portion within cracks to cause the formation of metal ions. The resulting positively charged metal ions hydrolyze in the surrounding electrolyte in the crack, increasing its acidity and hereby increasing the corrosion rate within the crack. in addition, the positively charged metal ions formed within the cracks attract some of the halide ions of the solution which accelerates corrosion therein. In contrast, acids such as sulfuric and nitric alone are not operable in the present process since they require an impracticably long period of time, usually within the range of about 20 hours, before sufiicient corrosion of the metal within the crack occurs to allow detection by a conventional wetting penetrant.

The corrosive strength of the crack enlarging solution is determined largely by the difference in the rate of corrosion between the open surface portion of the metallic material and the surface portion within a crack therein, i.e., it should be sufficiently strong to corrode the surface portion within a crack but not the open surface portion of the material to any significant extent. Since the acidity or the composition of the solution is not significantly changed at the free surface portion, but the acidity of the solution does increase rapidly at the surface portion within a crack due to'a number of factors such as, for example, a lack of oxygen within the crack, fairly weak acidic crack enlarging solutions are satisfactory. The useful acidity and halide ion content of a crack enlarging solution generally falls within a specific range for a particular metallic material so that the cracks are enlarged or opened up in a reasonable period of time, generally within about 15 minutes. The solutions of stronger acidity and/or higher halide ion content are preferably used where cracks of very small depth must be detected since these cracks corrode at a rate slower then deeper cracks.

Specifically, for an iron base high temperature alloy such as 'A-286(AlSl 660), an aqueous acid solution useful in forming the crack-enlarging solution contains hydrochloric acid in an amount ranging from about 1 to about 20 percent by volume of the aqueous acid solution (i.e., about 0.1 to about 2 Normal) with the preferred hydrochloric acid concentration ranging from about 5 to about percent by volume of the solution (i.e., about 0.5 to 1.5 Normal).

For another group of bodies of metallic materials a crack enlarging solution containing an anodic inhibitor can be formulated so that the metal will exhibit what is generally known open surface portion of the metallic material so that it will exhibit the phenomenon of crevice corrosion, i.e., it should not be used in an amount which would passivate to a significant extent the surface portion within a crack in the material. Typical anodic inhibitors include sodium dichromate, sodium chromate, potassium chromate, potassium dichromate, sodium bicarbonate, borax, pyrophosphate and calcium carbonate. The specific anodic inhibitor used depends largely on the metallic material to be tested. In general the amount of inhibitor should be very small, of the order of a few tens of parts per million, and the lower the halide ion concentration, the lower should be the inhibitor concentration. Specifically, the concentration of the anodic inhibitor in this type of crack enlarging solution generally may range from about 0.000l Molar or lower to about 0.05 Molar. Usually, for a majority of this type of metallic materials, the halide ions in the crack enlarging solution are provided ,by a metal halide salt with the chloride ions being preferred.Representative of these salts are sodium chloride, potassium chloride, sodium bromide, potassium bromide, sodium iodide and potassium iodide. The salt should be used in the aqueous crack enlarging solution in an amount sufficient to provide enough halide ions to corrode the surface portion contained in the cracks so that theyare enlarged or opened up in a reasonable period of time, generally within about 15 minutes. However, the salt should not be used in an amount which would causethe crack enlarging solution to corrode the open surface portion of the metal to any significant extent. The concentration of the metal salt used to.provide the halide ions in this crack enlarging solution can vary widely depending largely upon the particular salt used and the metal being tested but it generally ranges from about 0.05 to about 2 Molar. In addition, for some metallic rnaterials where the crack enlarging solution contains an anodic inhibitor so that the metal will exhibit the phenomenon of crevice corrosion, the halide ions can be provided by a halogen acid provided the acid isv sufficiently dilute in the solution so that it does not corrode the open surface portion of the metal but does corrode the surface portion within a crack therein.

The crack enlarging solution of the present invention may contain an oxidizing agent to accelerate corrosion of the surface portion contained in the cracks of the metal body.

Representative of such oxidizing agents are potassium ferricyanide, cupric sulfate, sodium bromate and potassium permangenate. The particular oxidizing agent used depends largely upon the particular metal being tested. For example, potassium ferricyanide is particularly useful for iron-containing alloys. The amount of oxidizing agent used in the crack enlarging solution is determinableempirically and depends on the metallic material being tested. For those metallic materials which exhibit the phenomenon of crevice corrosion and utilize acidic crack enlarging solutions, the oxidizing agent may be used in amounts ranging up to about 4 grams per cc. of the aqueous acid solution with amounts of about 0.5 to about 2 grams per 100 cc. of the aqueous acid solution being preferred. On the other hand, for those metallic materials that utilize crack enlarging solutions containing an anodic inhibitor, the amount of oxidizing agent may range from about 0.0005 to about 0.01 Molar.

A wetting agent can be included in the present crack enlarging solution to increase its penetrating and wetting properties. Conventional wetting agents can be used and they may be anionic, cationic or nonionic. The wetting agent should not be decomposed or degraded in the solution to any significant extent. The amount of wetting agent used is not critical and may vary within a wide range depending on the increased penetration produced by a specific amount. Generally, satisfactory results are obtained with amounts of wetting agent ranging from about 0.01 to about 2 percent by volume of the solution. Representative of the anionic wetting agents are sodium salts of organic sulfonates, especially alkylaryl sulfonates such as the sulfonates of dodecylbenzene, as for example, disodium-4- dodecylated oxydibenzenesulfonate. Other representative anionic surfactants include sodium alkylnapththalenesulfonate, sodium N-methyl-N-oleyltaurate, sodium oleylisethionate and the sodium salt of sulfated nonyl phenoxypoly (ethyleneoxy) ethanol. Typical cationic surfactants include lauryltri-methylammonium chloride and octadecyltrimethylammonium chloride. Examples of nonionic surfactants are polyethylene glycol lauryl ether and tris (polyoxyethylene) sorbitan monolaurate.

The present process can be carried out in a number of ways. The crack enlarging solution may simply be applied on the surface area of the metal body to be tested in a conventional manner such as from an eye dropper or other conventional means, or if desired, the part to be tested may be immersed in the crack enlarging solution. After a period of time, which is determinable empirically, the solution may be removed from the surface by simply wiping it off with a cloth or paper such as Kimwipe. The residence period for the crack enlarging solution should be sufficient for it to enlarge or open up cracks so that they can then be detected by conventional liquid wetting penetrants. However, the crack enlarging solution should not remain on the surface of the metal being tested for a period of time so long as to corrode the open surface portion to any significant extent.

' Comparison tests can be made with a sample of the material being tested to determine the most suitable period of time for the crack enlarging solution to be in contact with the metal surface being tested. Specifically, a conventional penetrant may be applied to ,a sample of the metal body to be tested and detection of cracks made in a conventional manner. The same sample can then be treated according to the present invention initially with the crack enlarging solution and then with the same conventional wetting penetrant, for one or more specific periods of time and the cracks detected at the end of each time period.

Theinvention is further illustrated by the following example.

In the following example, A286(AlSl 660) an iron-base super alloy was used which is the type of metal that exhibits the phenomenon of crevice corrosion. This alloy is composed of Ni-26 percent, Cr-l5 percent, No] .3 percent, Fe-balance, Ti-2 percent, Al0.2 percent, and B-0.l5 percent. This metallicmaterial is a high temperature steel used in the aircraft industry as a disc alloy which is an application that undergoes considerable fatigue.

EXAMPLE A bar of the A-286 alloy, 8 inches long, 0.5 inch wide and 0.125 inch thick was cut and a gage section was carefully surface ground in one direction. The bar was then fatigued by reverse bending to cause microcracks in the direction normal to that of the grinding direction. The bar had 3,750 cycles applied'where the total life is 12,500 cycles. These microcracks could be seen under a high powered microscope. (In normal operation, the cracks would not be distinguishable under a microscope since they usually occur in the same direction as the surface grinding marks).

A fluorescent liquid penetrant sold under the trademark HyRez type ZL-I-I which is a fluorescent dye in the form of Fluorol 7GA carried in a chlorinated solvent was applied with a swab to a surface of the gage section of the bar to form a continuous film thereon. After 10 minutes at room temperature, the penetrant was wiped off with I-IyRez Remover, Type ZR-H, on a Kimwipe paper and a I-IyRex ZP-HWA developer placed thereon. The bar was then placed under ultraviolet light and a photograph was made of the thus-treated gage surface. The photograph showed very few indications of cracks. The HyRez was then cleaned from the surface by wiping it off with Kimwipe paper.

A crack enlarging solution was then applied from an eye dropper to the same gage surface of the bar at room temperature to cover the entire surface being tested. The crack enlarging solution was formed by admixing 10 ml. of concentrated hydrochloric acid (38% conc.), 90 ml. of distilled water, one gram of potassium ferricyanide, and 0.1 ml. of a wetting agent sold under the trademark Aquet which is a nonionic alkyl aryl polyethylene glycol liquid. Within about 30 seconds a deep blue precipitate occurred at various locations in the surface indicating crack corrosion. After about 2 minutes, the blue precipitate was cleaned off by wiping the surface with Kimwipe paper. There was no visual evidence of any significant corrosion of the surface. The I-IyRez Type ZL-H penetrant was then applied with a swab to the same gage surface. After 10 minutes, it was wiped off with HyRez Remover Type ZR-H on a Kimwipe paper and the l-lyRez Type ZP-I-IWA developer was applied thereon. The bar was then placed under ultraviolet light and a photograph taken of this gage surface. It showed indications of a significantly greater number of cracks than was shown by the first photograph where there was no selective corrosion of the cracks.

In copending U.S. Pat. application Ser. No. 889,696 entitled Color Method For Detecting Cracks In Metal Bodies" filed of even date herewith in the names of Louis F. Coffin, Jr. and Lyman A. Johnson and assigned to the assignee hereofthere is disclosed a color method for detecting cracks in a metal body using a color-forming aqueous acid indicating solution which contains halide ions and a color-forming indicator.

ln copending US. Pat. application Ser. No. 889,699 entitled Anodic Inhibitor-Color Method For Detecting Cracks In Metal Bodies filed of even date herewith in the names of Louis F. Coffin, Jr. and Lyman A. Johnson and assigned to the assignee hereof there is disclosed a color method for detecting cracks in a metal body using a color-forming electrolyte which contains halide ions, a color-forming indicator and anodic inhibitor.

In copending US. Pat. application Ser. No. 889,693 entitled Pre-Passivation-Color Method For Detecting Cracks In Metal Bodies filed of even date herewith in the name of Michael F. Henry and assigned to the assignee hereof there is disclosed a process combining pre-passivation with a color method for detecting cracks in metal bodies wherein a colorforming aqueous acid indicating solution containing halide ions and a color-forming indicator is used.

In copending U.S. Pat. application Ser. No. 889,694 entitled Pre-Passivation-Anodic Inhibitor-Color Method For Detectmg Cracks In Metal Bodies" filed of even date herewith in the name of Michael F. Henry and assigned to the assignee hereof there is disclosed a process combining pre-passivation with a color method for detecting cracks in metal bodies wherein a color-forming electrolyte containing halide ions, a color-forming indicator and an anodic inhibitor is used.

All of the above cited patent applications are, by reference, made part of the disclosure of the present application.

What is claimed is:

1. A process for detecting cracks in a metal body which comprises applying to the surface of the metal body to be tested a crack enlarging solution to selectively corrode the surface portion contained in the cracks, said crack enlarging solution being selected from the group consisting of an aqueous solution of hydrochloric acid and an aqueous solution of I metal chloride and anodic inhibitor and being formulated so that it does not corrode the open surface portion of the metal body but is reactive with the surface portion within the cracks to enlarge or open up the cracks, said metal body being of the type which undergoes crevice corrosion upon application of said crack enlarging solution, after a period of time ranging up to 15 minutes removing said applied crack enlarging solution, and thereafter applying a liquid dye penetrant to said surface of the metal body, allowing said penetrant to flow into said cracks, wiping said metal body surface and detecting said cracks by means of the dye penetrant remaining therein.

2. A process according to claim 1 wherein said said crack enlarging solution is an aqueous solution of hydrochloric acid and said metal body is an austenitic steel.

3. A process according to claim 1 wherein the crack enlarging solution contains a wetting agent.

4. A process according to claim 1 wherein said crack enlarging solution is an aqueous solution of metal chloride and anodic inhibitor and said metal body is selected from the group consisting of iron, low alloy steels, medium alloy steels and iron-carbon alloys.

5. A process according to claim 4 wherein said metal chloride is sodium chloride and said anodic inhibitor is selected from the group consisting of sodium dichromate, sodium chromate, potassium chromate, potassium dichromate, sodium bicarbonate, borax, pyrophosphate and calcium carbonate.

6. A process according to claim 1 wherein said crack enlarging solution contains an oxidizing agent selected from the group consisting of potassium ferricyanide, cupric sulfate, sodium bromate and potassium permanganate.

7. A process according to claim 6 wherein said metal body contains iron and said oxidizing agent is potassium ferricyanide.

8. A process for detecting cracks in an iron-base super alloy metal body which comprises applying to the surface of the metal body to be tested a crack enlarging solution to selectively corrode the surface portion contained in the cracks, said crack enlarging solution being formed from about cc. of 5 to 10 percent by volume aqueous hydrochloric acid solution and about 1 to 2 grams of potassium ferricyanide, said solution being formulated so that it does not corrode the open surface portion of the metal body but is reactive with the surface portion within the cracks to enlarge or open up the cracks, after a period of time ranging up to 15 minutes removing said applied crack enlarging solution, and thereafter applying a liquid dye penetrant to said surface of the metal body, allowing said penetrant to flow into said cracks, wiping said metal body surface and detecting said cracks by means of the dye penetrant remaining therein.

9. A process according to claim 8 wherein the solution contains a wetting agent. 

2. A process according to claim 1 wherein said said crack enlarging solution is an aqueous solution of hydrochloric acid and said metal body is an austenitic steel.
 3. A process according to claim 1 wherein the crack enlarging solution contains a wetting agent.
 4. A process according to claim 1 wherein said crack enlarging solution is an aqueous solution of metal chloride and anodic inhibitor and said metal body is selected from the group consisting of iron, low alloy steels, medium alloy steels and iron-carbon alloys.
 5. A process according to claim 4 wherein said metal chloride is sodium chloride and said anodic inhibitor is selected from the group consisting of sodium dichromate, sodium chromate, potassium chromate, potassium dichromate, sodium bicarbonate, borax, pyrophosphate and calcium carbonate.
 6. A process according to claim 1 wherein said crack enlarging solution contains an oxidizing agent selected from the group consisting of potassium ferricyanide, cupric sulfate, sodium bromate and potassium permanganate.
 7. A process according to claim 6 wherein said metal body contains iron and said oxidizing agent is potassium ferricyanide.
 8. A process for detecting cracks in an iron-base super alloy metal body which comprises applying to the surface of the metal body to be tested a crack enlarging solution to selectively corrode the surface portion contained in the cracks, said crack enlarging solution being formed from about 100 cc. of 5 to 10 percent by volume aqueous hydrochloric acid solution and about 1 to 2 grams of potassium ferricyanide, said solution being formulated so that it does not corrode the open surface portion of the metal body but is reactive with the surface portion within the cracks to enlarge or open up the cracks, after a period of time ranging up to 15 minutes removing said applied crack enlarging solution, and thereafter applying a liquid dye penetrant to said surface of the metal body, allowing said penetrant to flow into said cracks, wiping said metal body surface and detecting said cracks by means of the dye penetrant remaining therein.
 9. A process according to claim 8 wherein the solution contains a wetting agent. 